1. Field of the Invention
The present invention relates to non-volatile semiconductor memories and, more particularly, to electrically erasable programmable read-only memories with large storage capacity.
2. Description of the Related Art
With the increasing needs for high performance and reliability of digital computer systems, the development of a semiconductor memory has strongly been demanded that has enough storage capacity to replace an existing non-volatile data storage device, such as a magnetic floppy disk unit, for computers. As compared with magnetic disk storage devices, such as a floppy disk unit and a hard disk unit, a presently available electrically erasable programmable semiconductor read-only memory is reliable and fast in data writing/reading speed. However, the electrically erasable programmable semiconductor read-only memory is not still large in data storage capacity enough to replace the above magnetic data storage devices.
In a conventional electrically erasable programmable read-only memory (abbreviated to "EEPROM" hereinafter), each memory cell is typically composed of two transistors and data is randomly written or erased one byte at a time. Hence, such high-density integration of the EEPROM as provides large storage capacity enough to replace the peripheral data storage devices will be difficult to expect.
Recently, as a non-volatile semiconductor memory which is integrated at high density and thus has a large storage capacity, an erasable programmable read-only memory has been developed that has a "NAND type cell" structure. This type of memory device is typically designed such that: (1) each memory cell utilizes one transistor having a floating gate and a control gate; and (2) a single contact is provided between an array of memory cells arranged on a substrate to constitute an "NAND cell structure" and a corresponding bit line. Hence, as compared with the conventional EEPROM, the area occupied by memory cells can considerably be reduced and thus the integration density can be improved.
The NAND cell type EEPROM suffers, however, from low operational reliability. Writing data into a desired memory cell selected out of memory cells is performed by discharging charges from the floating gate of a double gate FET corresponding to the selected cell. Erasing data is simultaneously performed on all the memory cells (called "simultaneous erase"), in which case the floating gates of all the cell FETs are simultaneously injected with charges. When there is a memory cell which is not to be written into but to be erased in a NAND cell block, the threshold value of this cell gradually undesirably increases after repetition of the data erase operation. According to an experiment conducted by the present inventors, it was confirmed that the threshold value of the double gate FET of a memory cell that was initially one volt increased to four volts or more when the number of repetitions of the erase operation was 10 and to as high as six volts for 100 repetitions of the erase operation.
Such variation of the threshold value of a memory cell FET which has repeatedly been subjected to an erase operation only will cause NAND cell type EEPROM to fail. That is, according to this type of EEPROM, in a data read mode, a supply voltage VCC is applied to the control gates of nonselected memory cell FETs in a specific NAND cell block including a selected cell so that the nonselected cell FETs are rendered conductive, while ground potential VS is applied to the control gate of the selected memory cell FET so as to determine whether potential that appears on a corresponding bit line corresponds to a logic 1 or a logic 0. Under this condition, if the threshold value of the nonselected cell FETs has increased as described above, correct data readout would become difficult. If the threshold value of the nonselected cell FETs increased to the supply voltage VCC or more, these FETs no longer could be turned on by application of the supply voltage VCC thereto. Therefore, the NAND cell type EEPROM will fail in effective data readout.